acid solution Any water solution that has more hydrogen ions (H+) than hydroxide ions (OH-); any water solution with a pH less than 7. Compare basic solution, neutral solution. 

advanced sewage treatment Specialized chemical and physical processes that reduce the amount of specific pollutants left in wastewater after primary and secondary sewage treatment. This type of treatment usually is expensive. See also primary sewage treatment, secondary sewage treatment.

basic solution Water solution with more hydroxide ions (OH2) than hydrogen ions (H1); water solution with a pH greater than 7. Compare acid solution, neutral solution.

biological oxygen demand (BOD) Amount of dissolved oxygen needed by aerobic decomposers to break down the organic materials in a given volume of water at a certain temperature over a specified time period. See dissolved oxygen content.

coastal zone Warm, nutrient-rich, shallow part of the ocean that extends from the high-tide mark on land to the edge of a shelflike extension of continental land masses known as the continental shelf. Compare open sea.

Cultural eutrophication Overnourishment of aquatic ecosystems with plant nutrients (mostly nitrates and phosphates) because of human activities such as agriculture, urbanization, and discharges from industrial plants and sewage treatment plants. See eutrophication. 

dredge spoils Materials scraped from the bottoms of harbors and streams to maintain shipping channels. High levels of toxic substances that have settled out of the water often contaminate these materials. See dredging. 

eutrophication Physical, chemical, and biological changes that take place after a lake, estuary, or slow-flowing stream receives inputs of plant nutrients--mostly nitrates and phosphates--from natural erosion and runoff from the surrounding land basin.See cultural eutrophication.

indicator species Species that serve as early warnings that a community or ecosystem is being degraded. Compare keystone species, native species, nonnative species.

neutral solution Water solution containing an equal number of hydrogen ions (H+) and hydroxide ions (OH-); water solution with a pH of 7. Compare acid solution, basic solution. 

nonpoint source Large or dispersed land areas such as cropfields, streets, and lawns that discharge pollutants into the environment over a large area. Compare point source.

Oxygendemanding wastes Organic materials that are usually biodegraded by aerobic (oxygen-consuming) bacteria if there is enough dissolved oxygen in the water. See also biological oxygen demand.

parts per billion (ppb) Number of parts of a chemical found in 1 billion parts of a particular gas, liquid, or solid. 

parts per million (ppm) Number of parts of a chemical found in 1 million parts of a particular gas, liquid, or solid.

parts per trillion (ppt) Number of parts of a chemical found in 1 trillion parts of a particular gas, liquid, or solid.

point source Single identifiable source that discharges pollutants into the environment. Examples are the (1) smokestack of a power plant or an industrial plant, (2) drainpipe of a meatpacking plant, (3) chimney of a house, or (4) exhaust pipe of an automobile. Compare nonpoint source.

primary sewage treatment Mechanical sewage treatment in which large solids are filtered out by screens and suspended solids settle out as sludge in a sedimentation tank. Compare advanced sewage treatment, secondary sewage treatment.

Secondary sewage treatment Second step in most waste treatment systems in which aerobic bacteria break down up to 90% of degradable, oxygen-demanding organic wastes in wastewater. This usually involves bringing sewage and bacteria together in trickling filters or in the activated sludge process. Compare advanced sewage treatment, primary sewage treatment.

septic tank Underground tank for treating wastewater from a home in rural and suburban areas. Bacteria in the tank decompose organic wastes, and the sludge settles to the bottom of the tank. The effluent flows out of the tank into the ground through a field of drainpipes.

sludge Gooey mixture of toxic chemicals, infectious agents, and settled solids removed from wastewater at a sewage treatment plant.

stream Flowing body of surface water. Examples are creeks and rivers. 

subsidence Slow or rapid sinking of part of the earth's crust that is not slope related. 

tertiary sewage treatment See advanced sewage treatment. 

water pollution Any physical or chemical change in surface water or groundwater that can harm living organisms or make water unfit for certain uses.